It is of no use for the artist to lift up his eyes with astonishment at any one being so insane as to question so generally admitted a statement. In vain does he point to his pictures, in which his greens have been actually so produced. The strict photologist at once puts him down, by informing him that he knows little or nothing of the real state of the case: his (the artist’s) colours are _negative_, or hues of more or less complete darkness; whereas in nature, the colour question is to be decided by _positive_ colours, or hues in which all the light used is of one kind. The meaning of this will be best understood by an example: When a ray of white light falls on a green leaf, part of the ray is absorbed and part reflected, and the object is therefore only seen with the part that is reflected. That which is absorbed consists of some of each of the colour rays, and the resulting reflected light is nothing more than a mixture of what remains after this partial absorption. The green we see consists of the original white light deprived of a portion of its rays. It is not a pure and absolute green, but only a residual group of coloured rays, and thus in so far the green colour is _negative_, or consists of rays not absorbed. It is therefore _partial darkness_, and not absolute light. If, however, on the other hand, a ray of white light is passed through a transparent medium (_e. g._ some chemical salt) which has the property of entirely absorbing all but one or more of the colour rays, and no part of the remainder, then all the light that passes through this medium is of the one colour, or a mixture of the several colours that pass: and if such light is thrown on a _white_ ground, the reflected colour will be _positive_, and not negative, and is far purer as well as brighter than the colour obtained in the other way. It has been found by actual experiment, that when positive blue, thus obtained, is thrown on positive yellow, the resulting reflected colour bears no resemblance to green. Sir John Herschel considers, that whether green is a primitive colour—in other words, whether we really have three or four primitive colours—remains yet an open question. It was necessary to explain these matters about colour before directly referring to the subject of this paper, namely, blindness to certain colour rays. It should also be clearly understood that the persons subject to this peculiar condition of vision have not necessarily any mechanical or optical defect in the eye as an optical instrument, which may be strong or weak, long-sighted or short-sighted, quite independently of it. Colour blindness does not in any way interfere with the ordinary requirements of vision, nor is there the smallest reason to imagine that it can get worse by neglect, or admit of any improvement by education or treatment.